The referenced U.S. Pat. No. 4,056,750 is a good historical survey over various ways to provide mercury in fluorescent lamps. The methods described in this reference are not suitable for high-speed mass production, and particularly high-speed automatic mass production. The patent describes a lamp which suitably has a shield band surrounding the filament, the shield band being formed with a gap. A metallic encapsulation element for mercury is so welded into the gap that the shield band is electrically closed. Upon application of a high-frequency current, by induction, the encapsulating element for the mercury is so heated that it will break and release the mercury.
It has been found that this arrangement is not entirely reliable to be suitable for mass production. Upon heating of the metallic capsule, contamination of the atmosphere within the lamp may result due to evaporation of material which adhered to the metallic encapsulating element.
The referenced U.S. Pat. No. 4,182,971 describes an elongated glass capsule for the mercury. A heating wire is placed axially through the glass capsule, extending therefrom at both sides. The glass capsule, again, is opened, or broken, by a high-frequency induction system.
This arrangement is difficult to make since introduction of mercury into the glass capsule is not easy. The heating wire must be melted into both ends of the glass capsule, which causes difficulties upon sealing the second end due to heat transfer through the wire to the end already sealed into the capsule. The heating may cause the mercury to develop a vapor pressure which interferes with the tight seal. Mercury may escape which, then, is missing in the lamp fill and its pressure.
The referenced U.S. Pat. Nos. 3,764,842, 3,794,402 and 3,794,403 describe a method and a lamp made in accordance with the method in which a closed glass capsule, retaining mercury, is pinched between an electrical conductor, namely the shield band, and the heating wire. In this arrangement the glass capsule must be additionally secured or retained in order to prevent uncontrolled rolling of the open glass capsule, or parts thereof, within the lamp bulb or tube. Any fragments which may be within the lamp bulb or tube may damage the filament and/or the fluorescent coating.